The use of mechanically adjustable balancing elements for this purpose is very restricted because their operation is troublesome and their disposition in apparatus of mini size is often not viable for reasons of space. Moreover, it is found impossible with such balancing elements to maintain the zero-balancing or zero-levelling over lengthy periods of time. It is an object of the present invention to provide a zero-balancing arrangement which avoids the drawbacks set out above. To this end, in the present invention, use is made of a differential amplifier with two inputs and a photoelectric element connected to each of the inputs, the output of the differential amplifier being connected to an indicator instrument, and an electronic regulating system connected to one of the inputs of the differential amplifier and adapted to come into operation on actuation of a switch to adjust the voltages set up at the two inputs of the differential amplifier to the same potential and the voltage at the output thereof leading to the indicator instrument to zero. In accordance with a further feature of the invention, the arrangement is advantageously such that the regulating system is connected to that input of the differential amplifier which is connected with the second photoelectric element located in the path of the comparison light beam.
In a preferred embodiment of the invention, the regulating system comprises a differential amplifier which is set up as an integrator, a condenser which feeds the integrated voltage, a Reed contact serving as a switch, and a feed circuit. The use of the feed circuit in the arrangement of the invention is very advantageous because the feed circuit maintains the zero-balance for a predetermined period of time after the Reed contact has been reopened.
The zero-balancing arrangement as devised in accordance with the present invention is used with particular advantage in apparatus of pocket form in which the constructional parts are incorporated in a flat casing which can be held in the hand and which includes at least one battery for a lamp used as the light source. In accordance with a further feature of the invention, the casing of the apparatus has an opening for introduction of a cuvet containing the measuring liquid into the path of the measuring beam, and a wedge filter is movable in the path by a manual operable setting wheel, the knurled periphery of which projects partially at the narrow side of the casing of the apparatus. Here a measuring scale is provided on the setting wheel and can be inspected through a viewing window at the flat side of the casing of the apparatus and is adjustable relative to a mark which is fixed in relation to the apparatus.